It has already been proposed to design the casing of such a vaporizer as a prismatic open-topped container of rectangular cross-section, the wick being a flat strip folded upon itself and received in the casing with its bight parallel to the major sides of the rectangle; the cover is of corresponding prismatic shape and has a height substantially equal to that of the casing. Reference may be made in this connection to U.S. design-patent application Ser. No. 763,661 filed Jan. 28, 1977 by Georg Schimanski.